Basic Information on the Pentax K mount:
Pentax K mount specifications
Mount type: Bayonet (lens release on body)
Flange focal distance: 45,46 mm
Film format: 36mm x 24mm (‘Full frame’)
Mount communication: see table below for details.
Adapting Pentax K lenses
If you’ve entered the photo scene during the 21st century, you might view Pentax as that company that’s maybe a contender for fourth or fifth place, lagging behind Canon and Nikon, and (lately) Sony. Then it might also surprise you, that for a big chunk of the latter part of the 20th century, Pentax was a contender for the top slot – and not only in terms of market share but in terms of technological leadership.
Pentax has undoubtedly been one of the central players in the industry since the Japanese reconstruction post WWII. Likewise, throughout its history, Pentax has been a very innovative company, always among the first to stretch the boundaries of (what was assumed to be) possible.
In contrast to many other long-lived, early 1 Japanese manufacturers (Canon, Minolta, Nikon, Olympus) Pentax (Asahi) did not early-on develop a proprietary mount, and instead used the M42 lens mount2. Subsequently, in 1975, Pentax launched its own bayonet mount (the Pentax K mount), that is still in use today.
I reiterate: You can go and buy Pentax’s newest dSLR marvel in 2022 and plug in a 1975 SMC Pentax 35/2 and the lens will function just as it was originally designed to do. That is – strictly speaking – something no other current dSLR can do (Nikon comes close, but current pro Nikon dSLR’s only support Ai/Ai-s Nikon lenses (1977->))3. The general rule of the Pentax K mount is “you can always use older lenses on newer bodies”. Hence, The Pentax K mount is also one that challenges the JAPB delineation of what makes a lens a legacy lens.
With close to 50 years under its belt, the Pentax K mount is – without a doubt – among the more venerable contenders, and in the field of SLR lens mounts, has really only one contender: the Nikon F mount. The exceptional longevity of these two mounts is based on that Pentax and Nikon were alone in successfully implementing a transition to AF lenses within the confines of the same lens mount (Minolta and Canon infamously orphaned their existing user base by switching to an incompatible mount).
At the same time – counting by the number of lenses made for a mount – the Pentax K mount has been among the most popular in history. Allphotolenses.com logs over one thousand different lenses made for the Pentax K mount – a number bested only by the Nikon F mount (and equalled by the M42 mount).
Another relatively striking feature of the Pentax K mount is that – for what essentially is a proprietary lens mount – it was used by a relatively broad range of companies. This is not only visible in the somewhat larger-than-normal share of third-party lenses for the Pentax K-mount, but also through the aspect that the Pentax K mount was used by a relatively broad field of Japanese camera manufacturers (whereas, e.g. only Canon made Canon FD bodies, and only Olympus made OM-system bodies). Moreover, some of these manufacturers further sold bodies for rebranding, leading to that Pentax K bodies can be found brandishing a wide range of marques.
Evolution of the Pentax K mount
Not surprisingly, considering the multitude of demands placed on a lens mount over a span of almost half a century of active innovation, Pentax’s dedication to the K-mount has lead to some solutions having been adopted, that clearly show an incremental nature.
A short genealogy of the Pentax K mount is repeated below (source: this article) with an eye towards adapting lenses to other mounts:
| Mount | Year | Comments | Features | Mount characteristics | Adaptability |
| K | 1975 | The original Pentax K | • Exposure: Av and M • Manual focus | • Two levers (aperture stop-down and aperture indicator) | MF: Problem-free |
| KF | 1981 | First autofocus mount. Quickly abandoned. | • Exposure: Av and M • Autofocus on one body- | • Two levers (aperture stop-down and aperture indicator) • 5 electronic contacts for operating AF | MF: Problem-free AF: Not available |
| KA | 1983 | Pentax K with electronic contacts for communication re: aperture range characteristics | • Exposure: Av, Tv, P, M • Manual Focus | • Two levers (aperture stop-down and aperture indicator) • 1–6 electronic contacts for communicating about aperture range. | MF: Problem-free |
| KAF | 1989 | First Pentax mainstream autofocus mount. KA functionality extended with: – autofocus facilitated by a drive shaft (slot-drive). – lens information through electronic contact. | • Exposure: Av, Tv, P, M • Auto-focus on compatible bodies • Lens info transmitted | • Two levers (aperture stop-down and aperture indicator) • 1–6 electronic contacts for communicating about aperture range. • 1 electronic contact to transmit lens info. • Slot-drive screw for AF | MF: Problem-free AF: Only with special adapter on limited systems 4 |
| KAF2 | 1991 | Two more electronic contacts added that enable lens-based autofocus. Originally these contacts were intended to enable power zoom. | • Exposure: Av, TV, P, M • Autofocus either with slot-drive or based on lens-internal motor • Lens info transmitted | • Two levers (aperture stop-down and aperture indicator) • 1–6+1 electronic contacts for communicating about aperture range. • 1 electronic contact to transmit lens info. • 2 electronic contacts to power AF. • Slot-drive screw for AF | MF: Problem-free AF: Only with special adapter on limited systems 4 |
| KA2 | 1997 | Like KAF2, but without AF drive shaft and without power contacts. Used by some third-party lenses and a manual-focus body. | • Exposure: Av, TV, P, M • No autofocus • Lens info transmitted | • Two levers (aperture stop-down and aperture indicator) • 1–6+1 electronic contacts for communicating about aperture range. • 1 electronic contact to transmit lens info. | MF: Problem-free AF: Only with special adapter on limited systems 4 |
| KAF3 | 2007 | Like KAF2, but without AF drive shaft | • Exposure: Av, TV, P, M • Autofocus based on lens-internal motor • Lens info transmitted | • Two levers (aperture stop-down and aperture indicator) • 1–6+1 electronic contacts for communicating about aperture range. • 1 electronic contact to transmit lens info. • 2 electronic contacts to power AF. | MF: Problem-free AF: Only with special adapter on limited systems 4 |
| KAF4 | 2016 | Like KAF3, but without aperture stop-down lever (electronic aperture) | • Exposure: Av, TV, P, M • Autofocus based on lens-internal motor • Lens info transmitted | • One lever (aperture indicator) • 1–6+1 electronic contacts for communicating about aperture range. • 1 electronic contact to transmit lens info. • 2 electronic contacts to power AF. | MF & AF: Only with special adapter on limited systems 4 |
First, off, adapting KAF4 lenses on any system does not work without using a specialised smart adapter.
For the entire rest of the spectrum (K–KAF2) adapting Pentax K lenses (as manual-focus lenses) follows the usual logic:
On dSLR’s, adapting with a simple ring (basically a dumb adapter) is possible to mounts that have a sufficiently shorter flange focus distance than the Pentax K mount (45,46 mm). Thus, e.g. using Pentax K lenses on Canon EF is relatively unproblematic. If the difference in flange focus distances is not sufficient for an adapter ring (such as is the case e.g. With the Nikon-F mount), one will have to use an adapter with optics. I am currently not aware of a Pentax-K to dSLR adapter that would allow for autofocusing any Pentax-K lenses.
On mirrorless cameras the story is, again different. Thanks to the generous difference in flange focal distances, one will never need an adapter with optics to address infinity focus. On the other hand, if the mirrorless system uses a smaller-than full frame sensor, a focal reducer might be useful. But assuming one is adapting to a full-frame mirrorless system (or feels no need for a focal reducer) all Pentax-K lenses up to KAF3 work flawlessly as manual focus lenses. Moreover, should one wish for autofocus, there are specialised adapters 4 even for that (with the exception of the KF variant).
Identifying the Pentax K mount
Footnotes:
1 From today’s angle, it is easy to overlook how many Japanese companies were active in the development, manufacture and sales of 35 mm SLR cameras during the 60’s. And while one in retrospect might assume that those camera companies that still are in the business (Canon, Fuji, Nikon, Olympus, Panasonic, Pentax, Sigma, Sony) were the ‘big ones’ even in the 60’s (in essence, assuming only the small ones have either been swallowed or gone out of business), that assumption is quite erroneous.
2 Especially in the US, the M42 is sometimes (erroneously) referred to as the Pentax Screw/Thread mount. I say erroneously, because Pentax neither developed the M42 mount (that happened in East Germany prior to WWII), nor were they ever the dominant M42 manufacturer. Pentax did, however, develop and market their own extensions of the M42 mount (read more here), and referring to that subtype of the M42 mount as some combination of “Pentax” and “M42” would be justified.
3 Nikon fans will be sure to point out that while the Pentax K mount is from 1975, the Nikon F mount is from 1959, and that many early Nikon F lenses have been factory converted to be Ai-compatible (and, indeed, I have a factory converted Nikon lens from 1965 that works on any new pro Nikon body), but that’s a bit beside the point.
4 There are third party adapters to enable AF on Pentax KAF/KAF2/KA2/KAF3/KAF4 lenses on some (body) systems, but (as of 2022) I recommend securing acquisition of the adapter before committing to purchasing lenses that need it.